1. Field of the Invention
This invention relates to composite material with high mechanical strength and excellent high-temperature characteristics. More particularly, it relates to composite material comprising a polymer matrix containing polyamide and layers of a silicate that constitute a clay mineral, said polymer matrix and layers of a silicate being bonded together and uniformly dispersed.
2. Description of the Prior Art
It has been widely attempted to admix inorganic materials, such as calcium carbonate and clay minerals (e.g. mica), to organic polymer materials in order to improve the mechanical properties of the latter. Admixture of these inorganic additives to a polymeric material, however, brings about many disadvantages, such as embrittlement of the polymer, because of the extremely weak interaction between additive and matrix polymer. The amount of inorganic materials that can be admixed is also very limited. Techniques are known in which these inorganic materials are previously treated with a silane coupling agent or the like to ensure higher affinity to matrix polymer. In this case, however, the organic and inorganic materials are present in separate phases and uniform dispersion of the latter cannot be expected. The result is insufficient reinforcing effect and limited improvement in high-temperature characteristics.
In order to overcome these problems, we formerly filed "Resinous Composition Containing Polyamide" (Japanese Laid-Open Patent Publication No. 83551/1982), which comprises a polymer matrix containing polyamide and flakes of vermiculite with an aspect ratio not smaller than 5 dispersed in said polyamide. This was intended to improve the mechanical strength of organic polymer materials by addition of vermiculite flakes with a large aspect ratio (length/thickness ratio of a particle). The resinous compositions obtained by this method show improved mechanical strength compared with conventional resins, but the difficulties are that sufficiently large aspect ratios cannot be achieved because mechanical crushing is indispensable to obtain flakes of vermiculite, and that a large amount of additive must be used to achieve necessary strength at the risk of embrittlement, because of the weak intermolecular bonding force between the mineral layer and matrix polymer.
Attempts have already been made to produce composite materials by synthesizing a polymer, such as polyamide and polystyrene, in the space between layers of a clay mineral. With conventional techniques, however, it has been difficult for the chains of synthesized organic polymer to fully penetrate between layers of clay mineral; hence, swelling of the interlayer space in the clay mineral is limited, resulting in imperfect dispersion of the silicate layers into organic matrix. This also entails a reduction in aspect ratio of the clay mineral, thus adversely affecting the effect of enhancing mechanical strength. In addition, the bonding between interlayer compound and matrix polymer is not sufficiently high. Consequently, satisfactory reinforcement cannot be achieved by this method.
The polyamides obtained in any of the above-mentioned methods have broad molecular weight distribution--the ratio of weight average molecular weight to number average molecular weight (M.sub.w /M.sub.n) is 6 or larger.
For some vinyl compounds, it is possible to prepare polymers of narrow molecular weight distribution by the living anion polymerization or by the group-transfer polymerization (Journal of the American Chemical Society 1983, 105, p. 5706). But these techniques are not applicable to polyamide.
Under the circumstances, we have continued systematic studies to solve the problems stated above, and succeeded in accomplishing this invention.